The present invention relates generally to control systems for manufacturing processes, and more particularly to a control system that uses Raman spectroscopy data to determine surface characteristics during formation of the surface.
Pulsed laser deposition is a method of applying a film to the surface of a substrate material. Often the film is specified to be very thin. An example of an application of pulsed laser deposition is the deposition of superconducting films on various substrates. The desired thickness of these films is in the range of approximately 100 to 50,000 angstroms.
During the pulsed laser deposition process, the substrate is placed in a vacuum chamber together with a solid mass of target material. This target material is often a composition of materials, which will undergo a chemical reaction during the deposition process, thereby forming the desired deposit material. A high power laser has its beam incident on the target material. The laser vaporizes the target material, producing a plume. The plume diffuses toward the substrate and is deposited on the surface of the substrate.
A problem with pulsed laser deposition is that it is difficult to reliably and consistently achieve a desired morphology, physic-chemical structure, and thickness of the deposited film. Other important properties that vary because of inadequate process control are current-carrying capability, surface roughness, and response to radiation. The lack of process control is exacerbated as the size of the surface to be coated increases.
As an example of poor process control, in the case of a superconducting film of yttrium barium copper oxide, manufacturing inadequacies account for a wide variation in critical temperature. The best films have a critical temperature of 92 K, but in actuality, most films have a lower critical temperature.
A particular problem has arisen with the demand for long length superconducting wires. Superconducting materials tend to be inflexible, thus superconducting wires are often formed by depositing a thin layer of superconducting material on tape made from a more robust material. However, attempts to manufacture long lengths of such tapes have not been able to overcome the existence of defects. Any break in the conductive path will cause the entire wire to be defective.
One aspect of the invention is a computer-implemented Raman-based spatial imaging method of using a computer to detect flaws in a surface. The method is xe2x80x9cspatialxe2x80x9d in the sense that data for an array of points on the surface is acquired and processed, rather than data for just a single point. To accomplish real time processing speeds, the data for each point represents only selected Raman peak frequencies rather than an entire Raman spectrum.
More specifically, the method makes use of stored reference data representing at least one Raman peak of a reference surface. In operation, an area of the subject surface is illuminated. For each Raman peak of interest, Raman response illumination received from the illuminated area is filtered, with the filtering corresponding to the frequency of that Raman peak. The filtered illumination is recorded as a camera image, which represents an array of Raman response data. This image can be mapped to the area of the surface. Additional images can be obtained for additional Raman peaks. If the response data is then compared to the reference data, and appropriate analysis performed, the thickness or chemical composition of the surface can be determined.